Systems and media for recording and reproducing audio information have evolved through several stages throughout history, including mechanical, magnetic and, most recently, optical technology. Advances in audio technology have led to improvements in the quality as well as the efficiency of audio systems. In the quest to develop the ideal recording and playback system, several features have been targeted for improvement, including quality and ease of reproduction, storage capacity, cost, durability and transportability of recording media, and overall system size. Therefore, although magnetic systems are years ahead of optical systems in terms of technical development, optical systems enjoy several advantages over their magnetic counterparts with respect to the above-identified features.
Compact discs (CDs) have introduced more people to digital audio systems than have any other medium. CDs contain digitally encoded audio information in the form of pits impressed onto their surface, which information is written to the CD using a recording system and later reproduced as sound using a playback system. While the reproduction accuracy of recording and playback systems for CDs has improved dramatically since their introduction to the consumer market during the early 1980s, a considerable amount of distortion still remains and although the amplitude of such distortion is relatively small, it is audible and distracting to audiophiles.
Much of the subject distortion is generated within the 16-bit analog-to-digital (A/D) and digital-to-analog (D/A) conversion circuitry, anti-aliasing circuitry and dithering circuitry of recording and playback systems and is attributable to several causes. First, 16-bit digital resolution, which is standard for consumer CDs, is inadequate for accurate reproduction of complex musical sounds. It is well known that at least 20 bits are required to achieve the accuracy and dynamic range needed to reproduce such complex sounds. As a result, a significant amount of quantization error is introduced when a sample of an analog signal is measured and assigned an approximate amplitude value.
A second cause of distortion results from attempts to reduce this quantization error. A technique referred to in the art as "dithering" involves the introduction of a small amount of white noise to an analog signal before the signal is digitally encoded. While dithering effectively expands the dynamic range of the system at very low signal levels, it does so at the expense of introducing a certain amount of nonlinearity and noise.
A final cause of distortion is the inadequate standard sampling rate for consumer CDs (44.1 kHz), which results in a significant ringing caused by the (sin x)/x components and phase distortion caused by the anti-aliasing filter circuits.
The decision to adopt 16-bit digital resolution and a 44.1 kHz sampling rate as standard for consumer CDs was made at a time when the available technology and price constraints dictated a compromise acceptable to a variety of international entities with competing interests. At the time, many of the interested parties were of the opinion that only 12 bits and 20 kHz were necessary to satisfy consumer demands. More recently, however, it has become obvious that the present standard is inadequate.
Many attempts have recently been made to solve the above-described distortion problem. For example, different types of D/A converters have been developed which use very sophisticated digital processing technology to improve the linearity of the standard 16-bit/44.1 kHz format. However, all such digital converters, while offering some audible improvement in smoothness, detail resolution and linearity at low signal levels, remain expensive to implement and fall far short of meeting current consumer demands.
Therefore, what is needed is an improved means for correcting distortion in a CD recording and playback system that does not greatly increase the cost of the system.